1. Field
The present disclosure relates to a process for the preparation of the diene elastomer functional along the chain which consists of a radical grafting reaction of functional groups on the unsaturations of the diene polymer in the presence of a radical initiator.
2. Description of Related Art
Various strategies are possible in order to modify the properties of the synthetic elastomers present in rubber compositions for tyres. One of the methods employed among these is the introduction of novel chemical functional groups at the end of or along the polymer chain.
The Applicant Companies are interested more particularly in the context of the invention in the functionalization along the diene polymer chain. Various types of reactions on the unsaturations of diene polymers which make possible the functionalization are known in the literature. Mention may be made of [4+2] cycloaddition reactions of Diels-Alder reaction type between a dienophile (for example maleic anhydride) and diene copolymers having conjugated dienes along the chain by virtue of the insertion of a conjugated triene comonomer (allo-ocimene) during the anionic copolymerization (EP 2 423 239 A1).
1,3-Dipolar cycloaddition reactions in the presence of nitrile oxide or nitrone (R. Huisgen, Angew. Chem. Int. Ed., 1963, 2, 565-632; R. Huisgen, Angew. Chem. Int. Ed., 1963, 2, 633-645; J. J. Tufariello, in 1,3-Dipolar Cycloaddition Chemistry, edited by Padwa, A., Wiley—Interscience, New York, 1984, Chapter 9, p. 83; K. B. G. Torssell, Nitrile Oxides, Nitrones, and Nitronates, VCH Publishers Inc., New York, 1988; K. V. Gothelf and K. V. Jorgensen, Chem. Rev., 1988, 98, 863-909) are also known for the functionalization (WO2012007441A1, WO2006045088A2) or the crosslinking of diene polymers (FR 1 583 406, WO2006081415A2).
The radical grafting of functional or nonfunctional thiol via photochemical or chemical catalyses (with or without radical initiator) forms part of these reactions for the functionalization of diene polymers (natural and synthetic rubber) in the same way as the cycloaddition reactions mentioned above (Angew. Chem. Int. Ed., 2010, 49, 1540-1573; J. Polym. Sci.: Part A: Polym. Chem., 2004, 42, 5301-5338; Polym. Chem., 2010, 1, 17-36).
European Patent Application EP 1 000 971 A1, filed by Bayer, presents the postpolymerization radical grafting of a carboxylmercaptan, such as 3-mercaptopropionic acid, to an SBR copolymer based on conjugated diene units on vinylaromatic units in the presence of a radical initiator, such as lauroyl peroxide. This chemical modification reaction, carried out in cyclohexane at 80° C. for 5 hours, leads to a grafting yield of 42%.
In the continuation of these studies, Patent Applications WO2009034001A1 and WO2009138349A1 describe a grafting process which consists in polymerizing butadiene by the anionic route and in adding, at the end of polymerization, 3-mercaptopropionic acid in the presence of lauroyl peroxide to the living polybutadiene chains before neutralization. The grafting yields obtained are approximately 70%, whether for an SBR having a high vinyl content (80%) or for an SBR having a low vinyl content (22%).
Applications WO200930840 and WO200930841 describe grafted polymers obtained from the reaction between copolymers based on conjugated diene units and on monovinylaromatic hydrocarbon units and thiol derivatives. The grafting reaction takes place in a reactor, under an inert nitrogen atmosphere, in the presence of a solvent, such as toluene, at a temperature of 90° C., with stirring, for a period of time of 3 to 4 hours, in the presence of a polymer, of a thiol derivative, which acts as graft, and of a radical initiator, such as azobisisobutyronitrile (AIBN). The grafting yields obtained under these reaction conditions are 39%.
In the continuation of these studies, Patent FR 2 962 440 A1 describes the grafting reaction between the diene polymer and a thiol derivative, in particular a paraffinic thiol derivative, in the absence of solvent and in the absence of any external radical initiator. The grafting yield obtained under these conditions is 71%.
The group of Prof. Boutevin describes the grafting of fluorinated thiols to hydroxyl telechelic polybutadiene oligomers (HTPB comprising 20% or 80% of 1,2-PB) at reflux of THF in the presence of AIBN or tert-butyl peroxypivalate (J. Polym. Sci., Part A: Polym. Chem., 1993, 31, 2069-2080). The authors demonstrated that, despite the presence of a large excess of thiol ([RSH]/[total double bond] initial molar ratio=4), the grafting yield is limited to 50%, in particular with regard to an HTPB having a low content of 1,2-PB. Nevertheless, the authors indicate the possibility of consuming the residual thiol by carrying out a second addition of peroxide after a time “t”.
In the light of the above, it appears that none of the radical grafting processes described makes it possible to obtain a quantitative grafting yield (100%). This disadvantage requires that the polymer be purified (precipitation from an alcohol, removal under vacuum or by steam distillation), as indicated in the various examples of the abovementioned patents, in order to prevent the presence of an ungrafted free entity. In particular, the use of a functional thiol molecule with a high boiling point or of thiol-terminated oligomer/polymer type would make it difficult to purify the polymer in the case of a nonquantitative yield.